1. Technical Field
The inventions are in the field of producing crude oil or natural gas and to pipeline transmission of oil or gas.
2. Background Art
Water-Soluble Polymers Used in Treatment Fluids
Common water-soluble polymers used in well treatment fluids include polysaccharides and synthetic polymers.
As used herein, a “polysaccharide” can broadly include a modified or derivative polysaccharide. As used herein, “modified” or “derivative” means a compound or substance formed by a chemical process from a parent compound or substance, wherein the chemical skeleton of the parent is retained in the derivative. Substitution is an example of a modification or derivatization process. Substitution on a polymeric material may be partial or complete.
A polymer can be classified as being single chain or multi chain, based on its solution structure in aqueous liquid media. Examples of single-chain polysaccharides that are commonly used in the oilfield industry include guar, guar derivatives, and cellulose derivatives. Guar polymer, which is derived from the beans of a guar plant, is referred to chemically as a galactomannan gum. Examples of multi-chain polysaccharides include xanthan, diutan, and scleroglucan, and derivatives of any of these. Without being limited by any theory, it is currently believed that the multi-chain polysaccharides have a solution structure similar to a helix or are otherwise intertwined.
Xanthan gum (commonly referred to simply as xanthan) is a polysaccharide, derived from the bacterial coat of Xanthomonas campestris. It is produced by fermentation of glucose, sucrose, or lactose by the Xanthomonas campestris bacterium. Diutan gum (commonly referred to simply as diutan) is another multi-chain polysaccharide that is sometimes used to increase viscosity in well fluids.
An example of a water-soluble synthetic polymer that is commonly used in wells is polyacrylamide or derivative of polyacrylamide. Certain polyacrylamides or derivatives can be classified as multi-chain polymers.
Potential Sources of Water for Use in Treatment Fluids
Non-freshwater sources of water for use in well treatment fluids can include surface water ranging from brackish water to seawater, brine, returned water (sometimes referred to as flowback water) from the delivery of a well fluid into a well, unused well fluid, and produced water. As used herein, a brine refers to a water having at least 40,000 mg/L total dissolved solids.
Another potential source of water for use in well treatment fluids can include push pills, that is, slugs of water that have been viscosified with a multi-chain polysaccharide used to push fluids to clean out an oil or gas transmission pipeline located at or near the surface of the ground or seafloor.
In some cases, however, a flowback water can have an undesirably high viscosity due to a residual viscosity-increasing polymer, which may or may not be cross-linked, that was not completely broken in the well before flowing back. Similarly, a push pill can have an undesirably high viscosity for use in a well treatment fluid. To use such a flowback water or push pill in forming another well treatment fluid, it may be necessary to break the residual viscosity.
Breaker for Polysaccharide or Crosslinked Polysaccharide
Reducing the viscosity of a viscosified fluid is referred to as “breaking” the fluid. Chemicals used to reduce the viscosity of fracturing fluids are called breakers. Other types of viscosified well fluids also need to be broken for removal from the wellbore or subterranean formation.
No particular mechanism is necessarily implied by the term “breaking.” For example, in the case of a crosslinked viscosity-increasing agent, for example, one way to diminish the viscosity is by breaking the crosslinks. By way of another example, a breaker can reduce the molecular weight of a water-soluble polymer by cutting the long polymer chain. As the length of the polymer chain is cut, the viscosity of the fluid is reduced. This process can occur independently of any crosslinking bonds existing between polymer chains.
Breakers must be selected to meet the needs of each situation. First, it is important to understand the general performance criteria of breakers. For example, in reducing the viscosity of a fracturing fluid or gravel packing fluid to a near water-thin viscosity, the breaker must maintain a critical balance. Premature reduction of viscosity during the pumping of the treatment fluid can jeopardize the treatment. Inadequate reduction of fluid viscosity after pumping can also reduce production if the required conductivity is not obtained.
Chemical breakers used to reduce viscosity of a fluid viscosified with a viscosifying polymer, such as guar and derivatized guar polymers, used in fracturing or other subterranean applications are generally grouped into three classes: oxidizers, enzymes, and acids. All of these materials reduce the viscosity of the fluid by breaking the polymer chain. The breakers operate by cleaving the backbone of polymer either by hydrolysis of acetyl group, cleavage of glycosidic bonds, oxidative/reductive cleavage, free radical breakage or combination of these processes. A breaker should be selected based on its performance in the temperature, pH, time, and desired viscosity profile for each specific treatment.
Fluids viscosified with a multi-chain polysaccharide can be more difficult to break than fluids viscosified with a single-chain polysaccharide. In particular, there are few methods available to break the fluid viscosity of a fluid viscosified with a multi-chain polysaccharide at low temperatures (below 120° F. or 49° C.), and they suffer from various problems. For example, the use of hypochlorite poses corrosion concerns and may not provide sufficient delay of the break. The current use of persulfate requires high concentrations at lower temperatures. The use of oxidizers such as sodium chlorite is limited to high-temperature applications and may react violently to cause a fire when reducing agents are used in the process. Enzymes do not work well on multi-chain polysaccharides such as xanthan at low temperatures.
Sodium perborate and ethyl acetoacetate (“EAA”) have been reported as being capable of breaking the viscosity of a fluid viscosified with a typical xanthan gum (“XANVIS”) down to 80° F. (27° C.). See Kelco Oilfield Group in its Technical Bulletin entitled “Breaker Applications,” revised January 2004. However, Halliburton previously reported that it was unable to break a fluid viscosified with xanthan at very low temperature using the published recipe and the publication does not provide sufficient detail to allow the user to optimize the breaker recipe for a given set of conditions. U.S. Patent Publication No. US 2008/0176770 A1, published Jul. 24, 2008, having for named inventors Michael W. Sanders, et al., which is incorporated by reference in its entirety.
A treatment fluid for use in a well can optionally comprise an activator or a retarder to, among other things, optimize the break rate provided by a breaker. Previously known examples of such activators include acid generating materials, chelated iron, copper, cobalt, and reducing sugars. Previously known examples of retarders include sodium thiosulfate, methanol, and diethylenetriamine.